The present invention generally relates to an electronic matrix system and method of operating the same. The present invention more particularly relates to a matrix system which includes a plurality of storage elements capable of storing electronic charge and a drive means and method for rapidly storing electric charge in selected storage elements and efficiently retaining the charge stored therein. The present invention is particularly useful in light influencing displays wherein the storage elements take the form of display picture elements and is also useful in other applications, such as in memory matrices.
In many electronic matrix systems, an array of storage elements, each having a unique address, are utilized for storing electric charge. Such matrix systems can include memory arrays and light influencing displays, for example. In light influencing displays, the storage elements take the form of picture elements. The picture elements or pixels generally include a pair of spaced apart and facing electrodes and light influencing material disposed between the electrodes. As a result, each pixel consititutes a capacitor in which electric charge can be stored. The charge stored in a pixel results in a voltage potential across the electrodes and an electric field across the light influencing material. By controlling the amount of charge stored, the properties of the light influencing material can in turn be controlled to obtain a desired light influencing effect.
When the light influencing material is liquid crystal material, alignment of the liquid crystal material molecules can be obtained when the field applied to the material is above a threshold value. As well known, liquid crystal displays generally include polarizers disposed on both sides of the display and alignment layers disposed on both sides of the liquid crystal display material. When the field across the liquid crystal display material is above the threshold value, a pixel can be made light transmissive or light absorptive depending on the relative alignment of the polarizers and alignment layers and when the field is below the threshold value, an opposite light influencing effect can be obtained. Because a display generally includes many pixels, an image can be formed by selectively controlling which pixels are transmissive to light and which pixels are absorptive to light.
In liquid crystal displays, it is necessary to update the condition of each pixel at regular intervals, for example, at a frame rate of thirty frames per second. This is required because the pixels can retain or store the applied potentials for a finite time. Updating is also required when nematic liquid crystal display material is employed because the sense of the applied potential must be reversed during alternate frames to avoid degradation of such liquid crystal display material. Updating is further required when the displayed images are intended to change regularly, such as when the displayed images are constantly moving. Hence, the ability to rapidly transfer to and store electric charge in the pixels and to efficiently retain the stored charge for at least one frame period is essential.
To accurately drive the pixels of liquid crystal displays, active matrices have been utilized. In such active matrices, each pixel is associated with one or more threshold devices through which the potential to be stored in the pixel is applied to the pixel. The threshold devices can take the form of field effect transistors or diodes, for example.
Active matrix liquid crystal displays employing diodes as the threshold devices are disclosed, for example, in copending U.S. patent applications Ser. Nos. 573,004, and 675,941 filed Jan. 23, 1984 and Dec. 3, 1984 respectively, in the names of Zvi Yaniv, Vincent D. Cannella, Gregory L. Hansell, and Louis D. Swartz, for Liquid Crystal Displays Operated By Amorphous Silicon Alloy Devices both of which are incorportated herein by reference. In accordance with at least one embodiment disclosed therein, each pixel includes a pair of diodes coupled in nonopposing relation between an address line pair and at a common node. One electrode of the pixel is coupled to the common node and the other electrode is coupled to another address line or data line to which the pixel charging potential is applied. A nematic liquid crystal display material is disposed between the electrodes. When charging potential is applied to a pixel, one diode is biased off and the other diode is biased on by potentials applied to the address line pair and to the data line. Hence, to charge the pixel, the charging potential is applied through one of the diodes. As a result, the nonlinear characteristics of the diode which is on must be overcome to provide the pixel with enough current to charge the pixel. The voltage drop across the diode must be overcome to provide this current and the charge potential must also be applied for sufficient time to accommodate the series resistance imposed by the diode. Also, during the next frame, the state of the diodes is reversed to accommodate the reversal in the charge potential polarity. This can cause the voltage at the common node to vary. Such variance in the potential at the common node presents difficulties in controlling the ultimate voltage to which the pixel is to be charged and can adversely affect gray scale operation of the display. Also, it can be understood that a reverse biased active device, such as a diode, for example, collects a certain amount of charge because of its internal capacitance. Under dynamic conditions, this unwanted charge must be taken into account when driving a pixel. Otherwise, an improper voltage will be applied across the pixel itself.
In addition to the foregoing problems, it is difficult to manufacture a very large area electronic matrix, such as a large area display, whose devices have precisely the same I-V characteristics over the entire length and width of the matrix. Thus, voltages applied to one pixel to achieve a particular gray scale effect or level may not product the exact same gray scale effect or level when applied to other pixels, especially those located some distance away from the one pixel.